Each vehicle’s measurement will be different, though most are generally somewhere between 0. 028-. 06 inch (0. 15 cm). You can either look in your owner’s manual or go to the auto parts store and have them look it up for you. Modified engines require smaller gap settings to account for the amount of power you’re putting through the engine. General rule of thumb: the more power, the smaller the gap.
A coin-style gap measure is usually the cheapest option, and works by running the edge of the “coin” through the gap until you reach a point where it stops. The edge is marked like a ruler, designating the thickness of the edge at that point. This is a good tool for older plugs you want to check the efficacy of, but can also inadvertently widen the gaps as you use it. A wire coin gap works essentially the same way as the regular coin-style gap, but features different wire measures of varying lengths on the edge around the coin. A blade-style feeler gauge is an effective and versatile tool. Built like a pocket-knife, this gauge features different blades of varying widths, some with wire on the end and some without, placed in the gap to check the spacing of the electrodes. You can also use multiple blades to measure larger gaps. These are effective to use while adjusting.
You can use a little bit of quick-drying (90%) alcohol on the contact points to clean them off if they’re especially dirty. [3] X Research source Lots of build-up or any blackening on the contact points might be a sign of a plug that’s outlasted its prime, however. If they’re especially dirty, you might consider buying new ones. There’s a special tool designed for cleaning spark plug—think of it as a sand-filled pouch that’s attached to an air compressor. It cleans the electrode of the spark plug by sandblasting it.
Most spark plugs and iridium plugs manufactured today don’t need to be gapped prior to installing them. If you’ve got a custom modified engine, though, you might be interested in checking or installing plugs at a different spacing. Adjust them accordingly. [5] X Research source
Never bend more than a few (. 02) fractions of an inch. It won’t take much pressure, so be very careful. Spark plugs can be quite delicate, and any fracturing can cause misfires. If you’re having trouble using the tool to adjust the gap, consider bending it in using a flat surface like a table to apply some very gentle pressure on the electrode to adjust it.
